The present invention relates to a method for setting black reference data for a image pickup device, and in particular, to a method for setting black reference data used for excess exposure, and an image reading apparatus.
Generally, a well known image reading apparatus projects light from a light source onto a document positioned on a document table, and read, as image data, light, that is reflected by the document, or that passes through the document, by an image pickup device.
The image pickup device used for this image reading apparatus comprises multiple pixels composed of charge coupling devices. An electric signal corresponding to the strength of the received light is output from the multiple pixels that constitute the image pickup device.
For the electric signal output from the image pickup device of the image reading apparatus, the variation in the sensitivity of the image pickup device or the variation in the amount of light emitted by the light source must be corrected. Therefore, the apparatus reads a black reference and a white reference before scanning a document, and prepares black reference data and white reference data. The thus prepared black reference data and white reference data are employed to perform shading correction for an electric signal output from the image pickup device.
An exposure time (also called a “charge accumulation period”) is determined for the image pickup device, so that the maximum value output from the image pickup device during the reading of the white reference is a predetermined value. This exposure time is a basic exposure time, and is set in accordance with the colors of light received by the image pickup device; such as, for example, red (hereinafter referred to as R), green (hereinafter referred to as G) or blue (hereinafter referred to as B). That is, the exposure time differs depending on the color of the light received by the image pickup device. Generally, a document is read based on the basic exposure time of the image pickup device. However, for a document such as a negative film, many low tone areas are included and the amount of R, G and B data is unbalanced. When such a document is scanned during the basic exposure time set for the image pickup device, a detailed, tonal expression of the dark portions is difficult, and the quality of the image that is read cannot be improved.
In order to implement the detailed tonal expression of the dark portions of the document, the gamma function may be changed to increase the output value. However, when the output value is increased by using the gamma function, the noise element included in the electric signal output by the image pickup device is also amplified. As a result, noise included in image data is amplified and the image quality is deteriorated.
To resolve this problem, excess exposure is employed whereby for the image pickup device the exposure time is extended beyond that for the basic exposure time, and the tones of the dark portion of a document are represented in detail. By using the excessive exposure time, the amplification of noise included in image data can be prevented, and accordingly, the deterioration of the image quality can be avoided.
However, the process for reading a document using the excess exposure does not take into account the exposure time required for the reading of a black reference. Therefore, the black reference data read during the basic exposure time is employed, regardless of the document type or the light received by the image pickup device. The black reference data is set, for example, by averaging the values of the dark portions output by the pixels that constitute the image pickup device.
For the black reference data set by averaging the output value in the above described manner, generally one value is employed for all the pixels, even when multiple pixels constitute the image pickup device. Therefore, to read a dark image, such as a negative film image, a problem has arisen in that the tones of a dark portion can not be expressed in detail for each pixel. In other words, when excess exposure time is employed, the tones of a dark portion included in a dark image can not be expressed by using the black reference data that is prepared based on the average black reference that is read during the basic exposure time.